Spin-Pumping Efficiency in Room-Temperature CdSe Nanocrystal Quantum Dots

Abstract

To understand and optimize optical spin initialization in room-temperature CdSe nanocrystal quantum dots (NCQDs), we studied the dependence of the time-resolved Faraday rotation signal on pump energy (Ep) in different sizes of NCQDs. The resulting spin-pumping efficiency (SPE) spectrum differs significantly from the absorption spectrum. In larger NCQDs, we observe two peaks in the SPE spectrum, whereas in smaller NCQDs, there is only one peak before a low, broad plateau. On the basis of our effective mass calculations, the Ep dependence of SPE arises from the competition between the heavy hole (hh), light hole (lh), and split-off (s.o.) band contributions to the optical transitions. At lower Ep, hh transitions dominate and give rise to the low-energy peaks. Because of the increasing contributions from the lh and s.o. bands which produce spin polarization in the opposite direction, SPE is reduced at higher Ep. The different number of peaks in larger and smaller NCQDs is attributed to size-dependence of the...